It is known to provide gas turbine engines with a fan nozzle throat area which can be varied to control fan working lines and reduce jet plume shock noise in cruise configuration and at take off. Variation in the fan nozzle area will allow tuning and control of the bypass flows in the engine for particular conditions that is to say at take off or during landing. It will be appreciated by varying the available area adjustments in the gas flow speed and orientation can be achieved.
Previous nozzle assemblies have been devised incorporating flaps or variations in the cowlings and nacelle surface used to vary the fan flow area through a gas turbine engine. In WO 03060312 use of a flap mounted in an area between a nacelle and an engine cowling is shown in which the flap moves inwardly and outwardly in order to vary the variable flow area. In JP 2001 050110 flexible tubes are located within the engine core cowling in order to expand into the gap between that cowling and a nacelle to again vary the available flow area.
These prior approaches have a number of disadvantages including a considerable increase in drag due to the formation of wake and recirculation regions behind the flaps. Such assemblies may also introduce flow unsteadiness due to flow separation within the bypass duct assembly between a nacelle and a core. There are also potential problems with regard to altering the aerodynamic loads, which can result in structural constraints or implications with regard to design. It will also be appreciated the potential utilisation of chevrons and other shaping to nozzle trailing edges means it may be difficult to incorporate flaps or flexible tubes as described above in order to vary flow area. With regard to use of elastic surfaces it will also be appreciated that the air and gas flows passing through a nozzle assembly may extend over a wide range of temperatures from below 0° C. to in excess of 100° C. and these temperature ranges may cause difficulties with respect to the operation of deflectable flaps or inflatable members.